Unmyelinated primary afferents from adjacent spinal nerves intermingle in the spinal dorsal horn: a possible mechanism contributing to neuropathic pain.

Peripheral nerve injury in animals can cause neuropathic pain often expressed in the form of hyperalgesia and allodynia. Spinal nerve ligation, in which the fifth and sixth lumbar (L5 and L6) or only the L5 spinal nerve is ligated and cut, is a model commonly used to produce neuropathic pain. The purpose of the present study was to test whether there is any anatomical evidence to support the suggestion that terminating unmyelinated (C) fibres of injured and adjacent uninjured nerves interact at the level of the spinal dorsal horn. Thus, in the first series of experiments, rats received injections of anterograde tracers, either wheat germ agglutinin (WGA) conjugated to horseradish peroxidase or Bandeiraea simplicifolia isolectin B4 (IB4), into the L4 or L5 spinal nerves. Results with both tracers showed that the central terminals of nerve L4 were concentrated in both L4 and L3 segments of the dorsal horn with clear although reduced levels of labelling in L2 and L5. Similarly, the central terminals of nerve L5 were found in both L5 and L4 again with less labelling in L3 and L6. These results suggest an intermingling of primary afferents of adjacent nerves at the level of the spinal dorsal horn. A second series of experiments was therefore conducted to test whether primary afferent terminals from adjacent nerves target the same neuronal elements in the regions of overlap. Consequently, additional rats were injected with WGA into the L5 spinal nerve and IB4 into the adjacent L4 spinal nerve. Double immunofluorescent staining and confocal microscopy revealed that IB4-labelled and WGA-labelled boutons, belonging to L4 and L5 spinal nerves, terminated in the same region within the L4 spinal segment. This suggests that neurons located in regions of overlap receive input from both L4 (intact) and L5 (injured) afferents. Consequently, spinal neurons located in regions of terminal overlap may show augmented responses to activation of the intact L4 nerve due to neuronal sensitisation resulting from injury to the adjacent L5 nerve. This may in part provide an anatomical basis for hyperalgesic reaction to injury.